Fig. 2.

The basic principle of photoacoustic imaging. When a tissue is exposed to pulsed near-infrared laser light, the constituents of the tissue (e.g., water, lipids, collagen, hemoglobin, etc.) absorb light and undergo thermoelastic expansion, thereby causing ultrasound signals to emanate (the photoacoustic effect). Therefore, these laser-induced ultrasound signals (photoacoustic signals) can be detected using an ultrasound transducer, making photoacoustic (or optoacoustic) imaging possible. Each of these biological absorbers can be targeted by irradiating tissue at the corresponding dominant absorption wavelength. As such, using a tunable laser at the relevant wavelengths of interest enables the acquisition of multiple photoacoustic images that can be spectrally resolved for tissue composition to be assessed based on endogenous contrast. PA, photoacoustic.